In general, ceramic capacitors are produced according to the process mentioned below.
First, a plurality of sheet-like dielectric materials each of which is coated with an electrode material to be an inner electrode are prepared. As the dielectric material, for example, employable is a material consisting essentially of BaTiO.sub.3. Next, these sheet-like dielectric materials each coated with such an electrode material are laminated together, pressed under heat to make them integrate, and thereafter fired at from 1250.degree. to 1350.degree. C. in air to give a dielectric ceramic having therein inner electrodes. Next, outer electrodes are attached to the terminals of the dielectric ceramic and electrically connected to the inner electrodes by baking. As a result of this process, a monolithic ceramic capacitor is obtained.
Therefore, the material of such inner electrodes must satisfy the following conditions.
(a) Since the inner electrodes are fired along with the dielectric material, they must have a melting point higher than the temperature at which the dielectric material is fired and PA1 (b) The material of the inner electrodes cannot be oxidized even in an oxidizing, high-temperature atmosphere and cannot react with the dielectric material.
As electrodes that satisfy the above-mentioned conditions, there have heretofore been employed noble metals such as platinum, gold, palladium, silver-palladium alloys, etc.
However, these electrode materials are expensive, although they have excellent characteristics. As a result, the proportion of the cost of electrode materials to the total cost in producing monolithic ceramic capacitors reached from 30 to 70%, and, therefore, was the most essential factor in increasing the production costs of such monolithic ceramic capacitors.
Apart from such noble metals, base metals such as Ni, Fe, Co, W, Mo, etc. are known to have a high melting point. However, these base metals are easily oxidized in an oxidizing atmosphere at high temperatures and therefore cannot be used as electrodes in such monolithic ceramic capacitors. Therefore, if these base metals are desired to be used as inner electrodes in monolithic ceramic capacitors, they must be fired in a neutral or reducing atmosphere along with the dielectric material. However, if conventional dielectric materials are fired in such a neutral or reducing atmosphere, they are noticeably reduced and converted into semiconductors.
In order to solve this problem, for example, a dielectric material comprising a solid solution of barium titanate where the ratio of barium sites/titanium sites is more than the stoichiometric ratio, such as that disclosed in Japanese Patent Publication No. 57-42588, and a dielectric material comprising a solid solution of barium titanate with an oxide of a rare earth metal such as La, Nd, Sm, Dy, Y or the like added thereto, such as that disclosed in Japanese Patent Laid-Open No. 61-101459, have been proposed.
In addition, other dielectric materials modified to have a reduced temperature-dependent variation in the dielectric constant have been proposed. As examples, mentioned are the composition of BaTiO.sub.3 --CaZrO.sub.3 --MnO-MgO such as that disclosed in Japanese Patent Laid-Open No. 62-256422 and the composition of BaTiO.sub.3 --(Mg, Zn, Sr, Ca)O-B.sub.2 O.sub.3 --SiO.sub.2 such as that disclosed in Japanese Patent Publication No. 61-14611.
By using these dielectric materials, it has become possible to obtain dielectric ceramics that are not converted into semiconductors even when fired in a reducing atmosphere, and the production of monolithic ceramic capacitors having inner electrodes made of base metal such as nickel and the like has been realized.
With recent developments in electronics, small-sized electronic parts have become much more popular, and the tendency toward small-sized, large-capacity monolithic ceramic capacitors has become remarkable.
Given the situation, the development of dielectric materials having an enlarged dielectric constant and also thin dielectric layers is desired. Accordingly, there is a great demand for reliable dielectric materials having a high dielectric constant and having a temperature-dependent variation in the dielectric constant.
However, the dielectric ceramics to be produced from the dielectric materials as disclosed in Japanese Patent Publication No. 57-42588 and Japanese Patent Laid-Open No. 61-101459 comprises large crystal grains, even though the materials have a high dielectric constant. Therefore, if the dielectric ceramics are used to produce thin dielectric layers having a thickness of not larger than 10 .mu.m in monolithic ceramic capacitors, the number of the crystal grains in each dielectric layer is decreased with the result that it is difficult to improve the quality and the reliability of the monolithic ceramic capacitors comprising the layers. In addition, the temperature-dependent variation in the dielectric constant of the dielectric materials is large.
On the other hand, the dielectric material as disclosed in Japanese Patent Laid-Open No. 62-256422 has a relatively high dielectric constant, and the dielectric ceramic to be produced from the material comprises small crystal grains and has a small temperature-dependent variation in the dielectric constant. In the material, however, CaZr.sub.12 O.sub.3 and also CaTiO.sub.3 that are formed during baking easily give a secondary phase along with Mn and others. Therefore, it is difficult to improve the reliability of the material at high temperatures.
The dielectric material as disclosed in Japanese Patent Publication No. 61-14611 has a dielectric constant of from 2000 to 2800. Therefore, it is difficult to say that the material is suitable in producing small-sized, large-capacity monolithic ceramic capacitors. In addition, the material does not satisfy the requirement stipulated in EIA Standard, which is such that the variation in the electrostatic capacity of condensers at temperatures falling between -55.degree. C. and +125.degree. C. shall be within .+-.15%.
In order to make them applicable to automatic surface mounting, small-sized, large-capacity monolithic ceramic capacitors have a plated film of solder or the like over the outer electrodes formed by baking an electroconductive metal powder. Electrolytic plating is generally employed to form such a plated film.
In general, the electrodes to be formed by baking an electroconductive metal powder have fine voids. Therefore, if a monolithic ceramic capacitor with such outer electrodes is dipped in a plating bath so as to form a plated film on the electrodes, the plating liquid penetrates into the electrodes through their voids. As the case may be, the plating liquid often reaches the interface between the inner electrode and the dielectric ceramic layer. For these reasons, the dielectric materials mentioned above are problematic in that their reliability is lowered.